U.S. Pat. No. 7,207,054 issued to Richards et al. discloses a power supply for low noise block converter (LNB) as shown in FIG. 1 hereof, in which a power converter 10 converts an input voltage Vin into a regulated voltage Vreg for a linear amplifier 12, a reference voltage generator 14 provides a direct-current (DC) voltage Vref for the power converter 10 and the linear amplifier 12, a signal generator 16 provides an analog alternating-current (AC) tone signal superposed on the DC voltage Vref by switching a switch SW with a signal φ1, and the linear amplifier 12 generates an output voltage Vlnb from the regulated voltage Vreg by referencing to the DC voltage Vref, so that the output voltage Vlnb has a DC level equal to the Vref and is modulated by the analog AC tone signal. The DC voltage Vref is selected from a plurality of preset DC voltages according to the desired output voltage Vlnb, and is added with a preset amount ΔV to determine the regulated voltageVreg=Vref+ΔV.  [Eq-1]In this art, however, at different loading or selected output voltage Vlnb, the linear amplifier 12 has a constant dropout voltage
                                                        Vdropout              =                            ⁢                              Vreg                -                Vlnb                                                                                        =                            ⁢                                                (                                      Vref                    +                                          Δ                      ⁢                                                                                          ⁢                      V                                                        )                                -                Vref                                                                                                        =                                ⁢                                  Δ                  ⁢                                                                          ⁢                  V                                            ,                                                          [                  Eq          ⁢                      -                    ⁢          2                ]            and thus has higher power consumption and thereby poor efficiency.